1. Field of the Invention
The present invention relates to a pressurizing unit, a fixing device, and an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses such as a copier, a printer, a facsimile machine, and a multifunction product includes a fixing device for fusing a toner image on a recording medium. Among various technologies that have been proposed in relation to such a fixing device is a fixing device disclosed in Japanese Patent No. 3298354. The fixing device employs a belt-nip system in which a rotatable heating roller with an elastically deformable surface is brought into pressure contact with a pressurizing unit to form a nip portion. While a recording medium is passing through the nip portion, a toner image on the recording medium is pressed and heated, and is thereby fixed on the recording medium. The pressurizing unit elastically deforms a portion of the heating roller corresponding to an exit portion (hereinafter, “nip exit”) of the nip portion, from which a recording medium exits.
In the belt-nip system, however, the local deformation at the nip exit changes the surface speed of the heating roller, and thus, shift of an image on the recording medium tends to occur. In addition, because a nip portion having a small curvature is formed by pressing an elastic layer of the heating roller and locally deforming the elastic layer, a heavy load is imposed on the recording medium, which increases an amount of damage on the recording medium or curling of the recording medium Reference may be had to Japanese Patent Application Laid-open No. 2005-164721.
A roller-nip system is also known, in which both ends of a metal core of a pressurizing roller is pressurized to form a nip portion. Whereas, in the belt-nip system, a pressurizing unit pressurizes a stationary member, which prevents an endless member from deviating.
A typical fixing device includes a heating unit and a pressurizing unit. A pressurizing unit of the belt-nip system includes a biasing member, a stationary member, a holding member, a pressure-contact member, and an endless member that is pressurized by the pressure-contact member. The biasing member presses the stationary member to form a nip portion between the pressurizing unit and the heating unit. If the mounting error occurs between the stationary member and the holding member, a direction in which the biasing member presses the stationary member can be different from a direction in which the holding member presses the pressure-contact member that pressurizes the endless member. Accordingly, sufficient nip pressure cannot be obtained. In addition, because the biasing member directly presses the stationary member, the stationary member may be damaged.
In most of the roller-nip systems, the whole outer surfaces of both ends of a metal core of a pressurizing roller is pressurized, so that the direction in which the both ends of metal core are pressed is substantially the same as the direction in which the pressurizing roller pressurizes the heating roller at the nip portion. Therefore, an appropriate nip portion can be formed. Whereas, in the belt-nip systems in which an endless member such as a belt is used, the direction in which the biasing member presses the stationary member is sometimes different from the direction in which the holding member presses the pressure-contact member, leading to a degraded image.
For removing a sheet by releasing nipping at the time of jamming, a pressure removing mechanism is required to release the pressure applied by the pressurizing member. The pressure removing mechanism, however, occupies space and increases the number of parts in the fixing device.